Studies have been made of the second-harmonic generation of optical maser beams in various crystals and investigators have elucidated the theoretical considerations underlying the general phenomenon of nonlinear diffraction with respect to second and higher order harmonic generation. For an early review of the subject of second harmonic generation, see P. A. Franken and J. F. Ward, Review of Modern Physics, Vol. 35, page 23 (1963). Additionally, one can refer to the following papers for discussions of various studies which have been made of second harmonic generation of optical maser beams in various crystals: Optical Harmonic Generation in Single Crystal BaTiO.sub.3 by R. C. Miller, Physical Review, Vol. 134, page A1313 (June, 1964); Critical Harmonic Scattering in NH.sub.4 Cl by I. Freund, Physical Review Letters, Vol. 19, No. 22, page 1288 (November, 1967); and Long-Range Order in NH.sub.4 Cl by I. Freund and L. Kopf, Physical Review Letters, Vol. 24, No. 18, page 1017 (May, 1970). See, in particular, Nonlinear Diffraction by I. Freund, Vol. 21, No. 19, page 1404 (November, 1968) which describes experiments in which the far-field pattern of second harmonic radiation has been recorded through a filter selectively transmitting such second harmonic. Nonlinear diffraction effects are of particular significance with respect to the investigation of structure of a crystal which has a spacially uniform linear susceptibility, but with a periodic spacial modulation of the nonlinear susceptibility. With such a crystal, no diffraction of light traversing the medium occurs, but harmonic generation, and particularly intense second harmonic generation, will occur at angles of incidence and diffraction for which the nonlinear analog of Bragg's Law is satisfied. An elementary theoretical description of such effect and experimental verification is given in the above paper by I. Freund entitled "Nonlinear Diffraction".
While the foregoing effects have been observed, it would be desirable to provide a method for microscopic examination of crystals displaying nonlinear diffraction. In particular, it would be desirable to provide apparatus and a method for microscopic examination for crystals which as a result of optical isotropy show no structure under ordinary microscopes.
The present invention provides such microscope apparatus and method for examination. Specifically, an optical harmonic microscope assembly and method is provided whereby lased coherent light is focused as an incident beam onto a specimen and the optical path of the microscope assembly and the incident beam is disposed at a relative angle in a range which is at least as broad as 0.degree.-90.degree. . The fundamental frequency of the lased light is blocked and the light of a harmonic of the fundamental frequency is viewed through the microscope. In a specific embodiment, a lased beam of infrared light is focused on a specimen supported on a microscope stage which can be tilted relative to the optical path of the microscope in a range of 0.degree.-180.degree. . The green second harmonic generated by nonlinear diffraction forms the image which is viewed through a filter which blocks the infrared light.
A polarizer can be incorporated in the light path, and further, the lased light can be polarized prior to impingement onto the specimen. The result is the visual discernment of crystal structure which would not otherwise be apparent. Specifically, polarization of the harmonic image reveals details of orientation of microcrystallites that are unavailable from ordinary microscopy.
The invention will be better understood when details thereof are considered in conjunction with the appended drawings.